Environmental concerns have led to continued efforts to reduce the NOx emissions of compression ignited (diesel) internal combustion engines. The latest technology being used to reduce the NOx emissions of diesel engines is known as exhaust gas recirculation or EGR. EGR reduces NOx emissions by introducing non-combustible components (exhaust gas) into the incoming air-fuel charge introduced into the engine combustion chamber. This reduces peak flame temperature and NOx generation. In addition to the simple dilution effect of the EGR, an even greater reduction in NOx emission is achieved by cooling the exhaust gas before it is returned to the engine. The cooler intake charge allows better filling of the cylinder, and thus, improved power generation. In addition, because the EGR components have higher specific heat values than the incoming air and fuel mixture, the EGR gas further cools the combustion mixture leading to greater power generation and better fuel economy at a fixed NOx generation level.
Diesel fuel contains sulfur. Even “low-sulfur” diesel fuel contains 300 to 400 ppm of sulfur. When the fuel is burned in the engine, this sulfur is converted to SOx. In addition, one of the major by-products of the combustion of a hydrocarbon fuel is water vapor. Therefore, the exhaust stream contains some level of NOx, SOx and water vapor. In the past, the presence of these substances has not been problematic because the exhaust gases remained extremely hot, and these components were exhausted in a disassociated, gaseous state. However, when the engine is equipped with an EGR system and the exhaust gas is mixed with cooler intake air and recirculated through the engine, the water vapor can condense and react with the NOx and SOx components to form a mist of nitric and sulfuric acids in the EGR stream. This phenomenon is further exacerbated when the EGR stream is cooled before it is returned to the engine.
Concurrent with the development of the condensed EGR engine, there has been a continued effort to reduce the content of sulfated ash, phosphorus and sulfur in the crankcase lubricant due to both environmental concerns and to insure compatibility with pollution control devices used in combination with modern engines (e.g., three-way catalytic converters and particulate traps). In Europe, a lubricant meeting the ACEA E6 low SAPS specification must pass, inter alia, the “Mack T10” engine test, which measures performance in an engine having a high degree of cooled exhaust gas recirculation, and the resulting presence of an increased level of inorganic mineral acids
Salicylate detergents are known to provide detergency that is superior to that of phenate and sulfonate-based detergents. Because of this improved detergency, the use of a salicylate detergent allows for a reduction in treat rate, and corresponding reduction in the metal content of the lubricant contributed by detergent. Thus, salicylate detergents have been favored in the formulation of low SAPS lubricating oil compositions. It has been known to use a combination of a low base number (neutral) salicylate detergent and a high base number salicylate detergent (overbased) to allow the formulators to precisely balance detergency and acid neutralization capacity, at minimum ash levels. Calcium salicylate detergents are used most commonly due to a perception that magnesium-based detergents may be the cause of certain performance debits, particularly increased bore polishing, in various industry standard tests to which lubricants are subjected.
In formulating low SAPS lubricants for the ACEA E6 category, the amount of ash contributed by the calcium salicylate detergent(s), combined with the ash contributed by the ash-containing antiwear agents in the formulation, must remain below the 1.0 mass % ash content limitation of the specification. The need to meet this stringent limitation on ash level, and provide adequate detergency performance led formulators to reduce the level of detergent overbasing. However, this reduction in the amount of overbasing reduces the acid neutralization capacity of the lubricating oil contribution. Lubricants containing reduced levels of detergent overbasing were found to provide unacceptable top-ring weight loss, and to a lesser extent, cylinder liner wear, in the Mack T10 test. While not wishing to be bound to any specific theory, it is believed that these performance problems are due to acid corrosion in the top-groove area of the engine piston.
Therefore, it would be advantageous to identify low SAPS lubricating oil compositions that better perform in diesel engines, particularly diesel engines equipped with EGR systems. Surprisingly, it has been found that by selecting certain detergent combinations and introducing relatively small amounts of compounds containing molybdenum and sulfur, low SAPS lubricating oil compositions demonstrating excellent performance in diesel engines, including diesel engines provided with EGR systems, can be provided.